The invention relates to a method of constructing a channel mask for an autostereoscopic display, the display comprising an electronically controlled screen covered by a parallax filter device that includes a refractive medium and is configured to obscure certain areas on the screen for a left eye of a viewer and to obscure certain other areas on the screen for a right eye of the viewer, the channel mask being a two-dimensional geometric object that permits to assign one of a number of pre-defined viewing positions of an eye of the viewer to each point on the screen.
According to the general principles of stereoscopy, an impression of spatial depth is generated by presenting to the two eyes of a viewer two different images that show the same scene from slightly different perspectives which represent the parallax difference between the left right and eye of the viewer.
Conventional systems for presenting different images to the left and right eyes of the user employ headsets or shutter glasses which, however, are quite disturbing for the user.
U.S. Pat. No. 8,077,195 B2 describes a system which permits to view autostereoscopic images “with the naked eye”, so that stereoscopic images can for example be produced on a screen of a computer monitor or the like. To that end, the image information of the left and right channels, i.e. the information intended for the left and right eye, respectively, of the user, is displayed on the screen in the form of segments, i.e. vertical or slanted stripes, which alternatingly belong to the left and to the right channel, and a parallax filter device, e.g. in the form of a lens array of cylindrical lenses is arranged in front of the screen and is carefully positioned relative to the pixel raster of the screen, so that the light emitted from the various screen pixels is deflected such that, for a specific position of the viewer, the information of each channel is visible only for one eye. A head tracking or eye tracking system may be employed for adapting the image representation on the screen to changing positions of the viewer. When a specific viewing position has been determined, a channel mask is constructed in order to appropriately assign the correct channel to each pixel.
The concept of channel masks may be extended to multi-view systems wherein the screen can be watched by one or more viewers from a number of different view positions. Then, the channel mask will define three or more channels, one for each of the envisaged viewing directions, i.e. the envisaged positions of an eye of a viewer.
The channel masks may be defined in an object plane, i.e. the plane that forms the surface of the screen, and in a principal plane of the parallax filter, which principal plane is somewhat offset from the object plane towards the viewer(s). For example, in case of a parallax filter in the form of an array of cylindrical lenses, the principal plane may be the plane that contains the apex lines of the cylindrical lenses.
When a viewer watches an area of the screen at right angles, one half of each cylindrical lens will deflect the light from the underlying screen pixels towards the left side of the user's face, and the other half of the lens will deflect the light of the underlying pixels towards the right side of the user's face. Consequently, the alternating pattern of apex lines of the cylindrical lenses and border lines between adjacent lenses will naturally define a channel mask in the principal plane, and a corresponding channel mask in the object plane can simply be obtained by an orthogonal projection in the direction normal to the screen.
However, if the screen is viewed under a certain skew angle, the skew angle being defined as an angle between the line of sight from the viewer to a point on the screen and a normal to the screen at this point, then, for this area of the screen, the channel mask in the object plane will be laterally offset relative to the channel mask in the principal plane. The amount of this offset will also be influenced by the refraction of the light rays at the apex of each lens.
Since the skew angles under which the screen is seen will generally be relatively small, the effect of the refraction can be compensated with reasonable accuracy by defining an auxiliary object plane between the object plane and the principal plane, the position of this auxiliary object plane being determined by the ratio between the refractive indices of the surrounding medium (air) and of the glass forming the lens, such that the channel mask in the object plane can be obtained by a central projection of the channel mask in the principal plane onto the auxiliary object plane, with the viewing position as projection center.